This invention relates to ship propulsion systems, and more particularly to an improved propulsion system control for a brake operable on the propeller shaft during initial transition from moderate or high forward ship speed to astern travel.
A common form of marine propulsion system employs ahead and astern air actuated clutches for connecting the prime mover to a reversing reduction gear unit for the propeller. The air actuated clutches are engaged by inflation and the degree of clutch engagement can be controlled by controlling the amount of inflation. A pneumatic control system is normally provided for controlling the amount of inflation and this control system also typically provides control for an engine speed governor which determines the engine speed. A single throttle lever apparatus can be provided for controlling both the clutch engagement and engine speed by movement of the lever in an ahead or astern direction from neutral. An example of such a control for a ship's propulsion system is found in my earlier U.S. Pat. No. 3,727,737, issued Apr. 17, 1973, for "Pressure Modulating System for Reversing Clutches and Throttle Control."
In the system of my earlier patent, I provided a pneumatic clutch control assembly for a ship's propulsion system that was sequentially operated to regulate the inflation of ahead and astern air inflatable clutches and to also control the prime mover speed. The control assembly was actuated by a single throttle lever located on a pilot house control stand. Movement of the lever in one direction provided forward rotation of a propeller at a speed which increased with handle travel away from neutral. Movement of the handle in the opposite direction provided astern rotation of the propeller with speed increasing as the handle was moved farther from neutral. The center position provided a neutral setting in which the engine was disconnected from the propeller and no power was transmitted, although the engine continued to idle.
The single lever control of both direction and speed was accomplished in the following manner: As the lever was pivoted in either direction from neutral, air was supplied to a selector valve which selected one or the other of the ahead and astern clutches. Thereafter, and up to a first control pressure, air pressure proportional to the position of the lever away from neutral fed through a first valve to the clutch and thereby began inflating the selected clutch. During this time the engine would remain at idle speed. After a first control pressure was reached, the first valve was piloted and it connected a second path for air to the clutch. This second path had provision for an initial programmed rate of feed of air to the clutch through a choke valve so as to softly inflate the clutch. Upon reaching a second higher control pressure, full supply air pressure was connected to the clutch. After the first control pressure was reached, the continued inflation of the clutch was not dependent upon the position of the throttle lever.
When the air pressure within the clutch rose to a predetermined level, the control of my earlier patent piloted a governor valve which, in effect, connected the throttle lever control to the speed governor of the engine so that the pressure supplied to the governor directly corresponded to the position of the throttle lever and the speed could be controlled by movement of the throttle lever. The throttle lever setting determined only the final operating speed and direction and all intermediate steps of clutch engagement and inflation, and engine governor speed were handled automatically by the control system.
With the use of high speed engines for ship propulsion, a command from the typical control to change direction from ahead to astern, if attempted to be accomplished at high speeds, can create severe problems with respect to the engagement of the astern clutches at a time when the propeller shaft is rotating at a high speed in the forward direction. To protect the clutches, a brake is applied to the propeller shaft on a reverse reduction gear when a shift from ahead to astern travel is commanded. The purpose of the brake is to absorb heat and relieve the astern clutches of a portion of a heat load which would otherwise be imposed upon them during high speed maneuvers.
When such propeller shaft brakes have been used, the systems have functioned to apply the brake during a delay period in neutral when neither the ahead nor astern clutch are engaged. The disadvantage to such an approach is that it lengthens the time required to alter the ship direction from ahead to astern, and it creates a period during which the ship is not under full control.
In accordance with my invention, I provide a control for a propeller shaft brake which will be applied during the portion of the cycle in which the astern clutch is being engaged. The control of the brake is by internal pressure in the astern clutch and is limited to operation at high vessel speeds. Further, the control will function to begin the application of the brake before the application of the astern clutch under high forward vessel speeds to further increase the effectiveness of the brake.